The invention relates in general to evaporators and in particular to a new and useful vacuum evaporator comprising a device for producing a d.c. current glow discharge which is intended for treating substrate surfaces to be coated. Such a treatment may be useful as a prepatory measure for vapor deposition onto the substrate, or during the vapor deposition, or even for subsequent treatment of the deposited layer.
Cleaning of the substrate surfaces prior to the vapor deposition is an example of such treatments. Aside from the mostly chemical cleaning methods applied before the substrates are placed in the vacuum chamber, a cleaning by cathode sputtering immediately prior to the coating or even a continuous cleaning during the coating, particularly in the initial phase of deposition, have proved very satisfactory, because such cleaning results in unusually well adhering layers. In the process, the negative pole of a d.c. voltage source is applied to the substrates and the pressure of the residual gas is adjusted to obtain a mean free length of path of the gas molecules between two consecutive collisions in the gas space, equal to about 1/30 to 1/100 of the distance by which the substrates are spaced from the opposite wall of the container. In the cathode drop region having a thickness of about 20 mean free lengths of path and where almost the entire discharge potential difference drop occurs, the formed ions are accelerated toward the cathode, impinge on the substrates and remove, by sputtering, first the contaminating matter and then also the uppermost surface layers, with the atoms sputtered away diffusing into the residual gas of the vacuum chamber and being flushed out by a gas stream. In this process, the positively charged gas ions discharge on the substrate and the corresponding electric current must be carried away, so that the application of the method is substantially limited to a cleaning of electrically conducting substrates.
To apply the method to non-conducting substrates, it is known to replace the d.c. voltage source by a high-frequency source, however, this requires more expensive equipment. It has further been provided to enclose the insulating substrates into a net or to dispose them behind a grate to which a negative d.c. voltage is applied. This, however, cannot prevent a simultaneous undesirable contamination of the substrate surface with the metal of which the net or grate is made, since these metal parts effective now as a cathode are also subjected to sputtering and the metal atoms sputtered off are partly precipitated on the substrate surface. It is true that this might not be disturbing if metal layers are to be deposited on the substrates. However, if transparent layers, for example anti-reflection layers on lenses, are to be deposited, such an advance coating with a metal is unacceptable. For these reasons the cleaning of non-conducting substrates by cathode sputtering has not been too practical, in spite of the better adhesion of the layers obtainable in this way.
Instead, another cleaning method is frequently used operated with a geometrical arrangement quite differing from that described above, namely a glow discharge, with the potential being applied to the substrates which, along with the wall of the container, form the anode on which the electrons impinge. Here, contrary to a cleaning by cathode sputtering, the again negatively polarized cathode is spaced from the substrates by many mean free lengths of path. However, the electrons impinging on the substrate surface in accordance with this method only effect the desorption of weakly bonded adsorbed substances, and no sputtering of firmly adhering surface layers. With the glow discharge arrangement, the substrates are cleaned in a gentle way, however, as mentioned, with an only moderate effect. In addition, with this method again, metal is sputtered from the cathode (glow electrode) only; and, since the substrates are spaced from the cathode by many mean free lengths of path, relatively few metal atoms sputtered from the cathode reach the substrate surface. Thus with the glow discharge method, in many applications, the contamination of the surface is not perceptible.
Another method in which the substrates are treated by means of an electric discharge is known as the "ion plating" or "ion-supported vapor deposition". What is important in this method is not only the continuous removal of contaminating matter during the condensation of the layer through ion bombardment, but still more the fact that energy is transferred by the ions to the surface to be coated, whereby the adhesive strength is furthered. This energy transfer may be effected either directly by the impinging ions, or indirectly, if the energy is first transferred to colliding neutral molecules (of the residual gas atmosphere, or of the vapor of the material to be condensed) and therefrom to the substrate surface. Such an energy transfer may be desirable even for an after-treatment of a layer, for example, to make it harder and more resistant to external effects, or to induce a chemical reaction, for example, oxidation of the layer in a suitable reactive atmosphere.